Optical fibers are thin strands of glass capable of transmitting information containing optical signals over long distances with low loss of signal strength. In essence, an optical fiber is a small diameter waveguide consisting of a fiber core having a first index of refraction surrounded by a fiber cladding layer having a second lower index of refraction. So long as the refractive index of the core is sufficiently higher than the refractive index of the cladding, a light beam propagating along the core exhibits total internal reflection and is guided along the length of the core. Typical optical fibers are made of high purity silica, with various concentrations of dopants added to control the index of refraction of the core and the cladding.
Many optical materials exhibit different responses to optical waves of different wavelengths. One such phenomenon is chromatic dispersion (or simply “dispersion”) in which the speed of light through an optical medium is dependent upon the wavelength of the optical wave passing through the medium. Since the index of refraction for a material is the ratio of the speed of light in a vacuum (a constant) to the speed of light in the material, it therefore follows that the index of refraction also typically varies as a function of wavelength in these materials.
Optical fibers are frequently used in data networks (such as telecommunication networks) requiring high data transmission rates. In many networks, data is also transmitted over very long distances. As the distances over which data is transmitted increase, or the rates at which data is transmitted increase, chromatic dispersion presents obstacles to achieving error free performance. Specifically, in long distance transmission of optical signals such as from a laser, the laser bandwidth and the modulation used to encode data onto the laser beam results in a range of wavelengths being used to transmit the information. The combination of this range of wavelengths and the chromatic dispersion of the fiber accumulated over a distance results in pulse broadening or spreading. For example, at high data rates two adjacent optical pulses or wave fronts may eventually overlap each other due to chromatic dispersion. Such overlapping can cause errors in data transmission. The accumulation of chromatic dispersion increases as the distance the optical signals travel increases. For low speed signals, dispersion is not typically a problem as they may use a smaller range of wavelengths, resulting in a smaller range of delays in the transmission path, and they also have a longer time period in which to determine the state of each bit.
Attempts to compensate for chromatic dispersion include the use of dispersion compensating fibers, dispersion compensating optical waveguide gratings (e.g., fiber Bragg gratings), and a combination of both. Dispersion compensating fibers and dispersion compensating optical waveguide gratings introduce a negative chromatic dispersion with an equal and opposite sign to the accumulated dispersion in a fiber link.
Optical gratings suitable for dispersion compensation may include Bragg gratings, and long period gratings. These gratings typically comprise a body of material with a plurality of spaced apart optical grating elements disposed in the material. For example, a conventional Bragg grating comprises an optical fiber in which the index of refraction undergoes periodic perturbations along its length. The perturbations may be equally spaced, as in the case of an unchirped grating, or may be unequally spaced as in the case of a chirped grating. That is, a chirp is a longitudinal variation in the grating period along the length of the grating. The wavelength reflected in a Bragg grating is directly related to the period of the perturbation. Thus, in a chirped fiber grating, the reflected wavelength of the grating changes with the position along the fiber grating. As the grating period increases or decreases along a direction in the fiber grating, the reflected wavelength increases or decreases accordingly. Therefore, different spectral components in an optical signal are reflected back at different locations along the grating and accordingly have different delays. Such wavelength dependent delays may be used to negate the accumulated dispersion of an optical signal. Chirped gratings may be linearly chirped (having perturbations that vary in a linear fashion), non-linearly chirped, or randomly chirped.
Fiber Bragg gratings reflect light over a given waveband centered around a wavelength equal to twice the spacing between successive perturbations. The reflected wavelength is given by the Bragg Equation λ=2dn, where n is the effective index of the grating, λ is the reflected wavelength, and d is the distance between successive perturbations. The remaining wavelengths pass essentially unimpeded. The ability to pass some wavelengths in an unimpeded manner is desirable in optical filtering applications. In such applications, the frequency of the grating can be selected to reflect (i.e., filter) undesired wavelengths, while allowing desired wavelengths to pass.
Fiber gratings may be extrinsically chirped or intrinsically chirped. An extrinsic chirp refers to a chirp in the grating that is obtained by applying an external perturbation generating field to the fiber. For example, to create an extrinsically chirped grating, an external gradient, typically comprised of strain gradients or temperature gradients, is applied along the length of a non-chirped fiber grating, resulting in non-uniform changes in properties of the fiber grating, thus creating a chirp. An extrinsic chirp is valuable in that it may be applied to adjust the parameters of the grating, and it may be used to control the chromatic dispersion of a fiber Bragg grating.
There are disadvantages, however, in forming chirped gratings with an external gradient. The maximum range of chirping that can be achieved is limited in that relatively large gradients or strains are required to obtain a range of chirping. Such externally applied strains may have a negative impact on the reliability of the fiber, such as by causing the fiber to fracture. Thus, the maximum chirp rate that can be imposed on the grating is limited by the material properties of the fiber.
An intrinsic chirp refers to a chirp in the grating that has been incorporated into the fiber during the fabrication process. For example, an intrinsic chirp may be achieved by using a phase mask in which the period of the phase mask varies in some manner. When radiation is applied to the fiber through the phase mask (thus altering the index of refraction), the resulting fiber will be intrincally chirped. Using this technique, broadband gratings may be produced which can compensate for chromatic dispersion in multi-channel system. However, intrinsic chirp corrects a fixed amount of dispersion in a specified wavelength spectrum. While intrinsically chirped gratings are useful in communication systems where a specific amount of dispersion compensation is required, the dispersion and amplitude response of the grating is essentially fixed. Thus, intrinsically chirped gratings by themselves are not well suited to situations in which dynamically adjustable devices are required.
A variety of approaches exist for adjusting the spectral response (i.e., “tuning”) of gratings in optical fibers. The application of strain to the grating is one method. For example, simply stretching the grating by gripping the fiber on either end of the grating and then putting the fiber in tension. The act of imparting a tensile strain on the grating results in a proportional increase in the wavelength of the spectral response, as per the following equation   Δλ  =                    λ        max            ⁡              (                  1          -                      p            e                          )              ⁢                  Δ        ⁢                                  ⁢        L            L      where Δλ is the wavelength shift resulting from the imparted tensile strain, ΔL/L; λmax is the maximum wavelength; and pe is the strain optic coefficient of the grating (an intrinsic property of the fiber in which the grating is written). Other approaches use bending of a simple structural member to put a strain on an attached grating. Each of these approaches is limited in the applications it can address.
In particular, a problem with current devices and methods requiring the application of tensile strain to the grating is the potential for tensile failure of the fiber. This problem severely limits the amount of strain that can be reliably imposed. Methods wherein the fiber is attached to a support member and the support member is then put into tension are likewise limited.
Short gratings (less than approximately 100 mm in length) have also been tuned using axial compression. For example, a grating may be attached to the inside of a ferrule and the ferrule placed in compression. When using a uniform ferrule, the wavelengths reflected will shift uniformly downward. When using a non-uniform ferrule, both the dispersion characteristics of the grating and the center wavelength change with changes in the force applied to the ferrule. Since only compression is used, tensile failure of the grating ceases to be an issue. However, the length of grating that can be economically tuned using this method is very limited. The length is also limited by column buckling of the ferrule, which requires that, for a given strain, the diameter of a cylinder increases as a function of its length. Furthermore, the force required to generate a given strain increases with the square of the diameter. These factors limit the use of the described compression tuning methods for long gratings (greater than approximately 100 mm in length).
Another problem current devices and methods for tuning share is that the wavelengths shift in all sections of the grating at the same time, although not necessarily by the same amount. Designs have been attempted that independently tune small elements within a grating. These designs have generally not been commercially successful due to their great complexity, large number of parts and high cost. For example, U.S. Pat. No. 5,694,501 discloses an apparatus and method of controlling strain in a Bragg grating that requires a segmented piezoelectric stack with quasi-distributed voltage control. The segmented piezoelectric stack and control system are complex, and a constant supply of power is required to maintain a selected strain profile.
Clearly, previous attempts to provide dispersion compensating devices and methods have not produced adequate results. In particular, previous attempts have not satisfactorily addressed the unique issues associated with adjusting the spectral response of long gratings or sections of long gratings. If methods representing the current technology were used to adjust the spectral response of long gratings, the result would be severely limited ranges of adjustability, or unacceptably high levels of tensile strain (>1%) on the fiber for high reliability. Currently available devices and methods also do not provide the capability to tune the dispersion of a single channel (wavelength) of a multi-channel grating at any wavelength within a broad operating range. A need exists for an apparatus and method that addresses the above described shortcomings.